M. Sroka

Influence of long-term service on microstructure, mechanical properties, and service life of HCM12A steel

The investigated material was a pipe measuring φ38 × 6.3, made of HCM12A steel used for coils of boiler superheaters at supercritical and ultra-supercritical parameters of steam. The article presents the results of research on mechanical properties and microstructure of the examined pipe material in the as-received condition and after 100 000 h of service at the temperature of 585°C. Creep tests were carried out for the material serviced for 100 000 h to determine the available residual life. The influence of changes on mechanical properties and microstructure degradation on the loss of creep resistance was assessed compared with the as-received condition. Performed research has shown that the examined steel after 100 000 h of service fulfills the requirements set for this steel in the as-received condition within the range of strength properties at room and elevated temperature. The insignificant loss of life is confirmed by the results of creep test.

Microstructure and mechanical properties of the T23 steel after long-term ageing at elevated temperature

The paper presents the results of research on the microstructure and mechanical properties of T23 steel after long-term ageing up to 70 000 h at the temperature of 550 and 600 °C. It has been shown that the main mechanisms of degradation of the T23 steel microstructure were: recovery of the matrix, disintegration of the bainitic microstructure, growth of carbide diameter and precipitation of M6C carbides. These processes were more advanced in the steel aged at higher temperature. The changes in the microstructure resulted in a 10–15% decrease in mechanical properties (YS, TS) and hardness HV10, and a 30–40% decrease in the impact energy kV. A greater fall of impact strength occurred in the steel aged at the lower temperature. This was ascribed to the segregation of phosphorus to grain boundaries, which has a well-known adverse effect on ductility of low-alloy steels.

Estimation of long-term creep strength in austenitic power plant steels

This paper presents the characteristics of the performance of austenitic steel Super 304H and HR3C used for construction of boilers with supercritical and ultra-supercritical steam parameters. Characteristic microstructure images of the examined steels in the as received condition observed with the scanning electron microscope are shown. The method for strength assessment based on abridged creep tests carried out at a temperature higher than the design one is presented on the example of examined steels. It has been demonstrated that the obtained results do not deviate from the values of creep strength determined in long-term creep tests. The maximum difference is ± 20%, which is in compliance with the acceptable scatter band. The presented methodology can be used for verification of creep strength.

The effect of temperature on the changes of precipitates in low-alloy steel

Investigations on the microstructure of low-alloy steel in the as-received condition and after 70 000 h ageing at 550 and 600°C were carried out. For the recorded images of the microstructure, a quantitative analysis of precipitates was performed. On that basis, a statistical analysis of collected data was made with the aim of estimating parameters of selected theoretical statistical distribution. Then, the forecast for average precipitate diameter and standard deviation of such a distribution for the time of 100 000 h at 550 and 600°C was calculated. The obtained results of investigations confirm the possibility of using, in evaluation of degradation degree for materials in use, the forecasting methods derived from mathematical statistics.

Microstructure and mechanical properties of HR3C austenitic steel after service

Purpose: The purpose of the investigations was to determine changes in the microstructure and mechanical properties of HR3C creep resisting austenitic steel after service. Design/methodology/approach: The investigations were performed on test specimens taken from a part of the steam superheater tube. The range of the investigations included: microstructural investigations light and SEM microscope; analysis of precipitates carbide isolates; investigations of mechanical properties hardness measurement, static tensile test, impact test. Findings: The precipitation processes at the grain boundaries lead to increase in intergranular corrosion of the HR3C steel resulting in loss of grains in the structure. The impact strength testing on test specimens with reduced width may result in overestimation of crack resistance of the material after service. Research limitations/implications: The comprehensive analysis of precipitation processes requires TEM examinations. Finding the correlation between the impact strength determined on standard vs. non-standard test specimens with reduced width. Practical implications: The obtained results of investigations are used in industrial practice for diagnosis of pressure parts of power boilers. Test procedures developed based on comprehensive materials testing conducted under laboratory conditions are used in upgrading and design of pressure parts of steam boilers. The results of investigations are also the element of database of the materials characteristics of steels and alloys as well as welded joints made of them working under creep conditions developed by the Institute for Ferrous Metallurgy. Originality/value: The results and analysis of the investigations of microstructure and mechanical properties of HR3C steel after service under actual boiler conditions are presented.

The evaluation of suitability for operation of repair low-alloy steel welded joints

Purpose: The purpose of the work was to evaluate the material condition of repair welded joints of steam pipelines with regard to their suitability for operation including determination of the time of their further safe operation. Design/methodology/approach: The following was carried out: microstructure investigations using scanning electron microscopy, investigations on mechanical properties at room and elevated temperatures, determination of the fracture appearance transition temperature based on impact tests. The abridged creep tests without measurement of elongation during test were carried out to determine the material’s residual life. Findings: The time of safe operation was determined for welded joints made in 14MoV63 steel after long-term service and 14MoV63 steel after long-term service and in the as-received condition. Practical implications: The applied methodology and the adopted procedures are used in evaluating the condition and forecasting further operation of new repair welded joints for the pressure part components of power equipment working under creep conditions. Originality/value: The obtained results of investigations will be the elements of materials characteristics worked out by the Institute for Ferrous Metallurgy for the steels and welded joints made in them to work under creep conditions.